Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/383

 they have acquired their present abrupt form, and contracted their original dimensions; while in certain cases even their very remains may have disappeared, where peculiar or accidental circumstances enabled the river to act on them to the last extremity.

It will be readily asked why similar terraces are not found at the two upper lines, as well as at the lower one. The answer is not only easy, but tends to confirm the view of their origin here given. It must be recollected that the present state of the valley is essentially different from its condition under both the former cases. In both, the bottom of the valley, instead of being the course of a running stream, was a lake. Under such circumstances it is evident, that the effect of these waters on any deltas occupying such a relative position as the level which the lake last quitted, must have been to undermine and remove them had they existed to any extent; whereas, on the final drainage of the whole, they have necessarily been left in their original integrity, subject only to the gradual, corrosive, and always diminishing action of the river. But it must also be considered, that as the rapidity of the Roy and its consequent corrosive power, was much less considerable when the valley was full of water, and its fall was consequently less, a much less quantity of alluvial matter would have accumulated at its entrance during the existence of the two first lakes.

But indeed, although no conspicuous terraces are found at the level of the upper lines, there are sufficient indications of their former existence in many places; while in all they lie near the entrances of the torrents, and thus confirm the view which I have here given of their origin. I shall here also remark that this view explains those irregular appearances of lines unconnected with the principal ones, which I have, in the description, sometimes called supernumerary, and which occur here and there in positions intermediate to them.